1. Field of the Invention
The present invention relates to apparatus for measuring rotation rates about three orthogonal axes. More particularly, this invention pertains to a loop controller for use in a triaxial gyro of the type in which a multiplexing arrangement is employed for reducing system components.
2. Description of the Prior Art
The Sagnac interferometer is an instrument for determining rotation by measurement of the non-reciprocal phase difference generated between a pair of counterpropagating light beams. This instrument generally comprises a light source such as a laser, an optical waveguide consisting of several mirrors or a plurality of turns of optical fiber, a beamsplitter/combiner, a detector and a signal processor.
In an interferometer, the waves coming out of the beamsplitter counterpropagate along a single optical path. The optical waveguide is "reciprocal"; that is, any distortion of the optical path affects the counterpropagating beams similarly although they do not necessarily experience such perturbation at the same time or in the same direction. Time-varying perturbations may be observed where the time interval is comparable to the propagation time of the light around the optical waveguide whereas "non-reciprocal" perturbations affect the counterpropagating beams differently and according to the direction of propagation. Such non-reciprocal perturbations are occasioned by physical effects that disrupt the symmetry of the optical medium in which the two waves propagate. Two of the non-reciprocal effects are quite well known. The Faraday, or collinear magneto-optic effect, occurs when a magnetic field creates a preferential spin orientation of the electrons in an optical material whereas the Sagnac, or inertial relativistic effect, occurs when rotation of the interferometer with respect to an inertial frame breaks the symmetry of propagation time. The latter effect is employed as the principle of operation of a ring gyroscope.
It is known that the fringe or interference pattern formed by the counterpropagating beams of a gyro consists of two elements, a d.c. component and a component that is related (e.g. cosine function) to the cause of the phase difference between the beams. This phase difference provides a measure of the non-reciprocal perturbation due, for example, to rotation. As a consequence of the shape of the fringe pattern, when small phase differences are to be measured (e.g. low rotation rates), the intensity of the combined beam is relatively insensitive to phase difference as such difference occurs close to the maximum of the phase fringe pattern. Further, mere intensity of the composite beam does not indicate the sense or direction of rotation.
For the foregoing reasons, an artificially biased phase difference is commonly superimposed upon the counterpropagating beams. The biasing of the phase shift, also known as "non-reciprocal null-shift," enhances the sensitivity of the intensity measurement to phase differences. A maximum degree of sensitivity is achieved by shifting the operating point of the gyroscope to .+-..pi./2 (or odd multiples thereof). Furthermore, by alternating the bias between +.pi./2 and -.pi./2, two different operating points are observed. This enables the system to determine the sign of the phase difference and, thus, the direction of rotation.
In addition to phase modulation, the processing of an interferometer output commonly employs "phase hulling" that introduces an additional phase shift through a negative feedback mechanism to compensate for that due to the non-reciprocal (Sagnac) effect. Commonly, the negative feedback generates a phase ramp whose slope is proportional to the rate of rotation to be measured. In actual practice, a ramp whose height varies between 0 and 2.pi. radians is employed as the nulling phase shift cannot be increased indefinitely due to voltage constraints.
U.S. patent Ser. No. 4,705,399 of Graindorge et al. discloses a digitally-based arrangement that employs a "stairstep" waveform. The height of each step is equal to the measured phase difference while the width or period of each is the group delay time of the optical coil. On the average, the slope of the ramp is equivalent to the measured non-reciprocal phase difference per unit of time. This method is compatible with digital signal processing and enjoys many resulting advantages. The phase modulation may be directly added to the digital ramp through the synchronization offered by a digital signal processor. The (combined) signal ultimately controls the phase modulator that is positioned near one end of the optical fiber coil.
Many applications, including navigation, require rotation and position information with respect to the three orthogonal space axes. Accordingly, a triad of interferometers would then be required, one for sensing rotation about each of the rotation axes. The necessity of deploying multiple interferometers can greatly complicate the amount and complexity of associated signal processing electronics. One approach to simplifying the signal processing electronics and thereby reducing the cost of a triaxial fiber optic gyroscope is taught in U.S. patent Ser. No. 5,033,854 of Matthews et al. entitled "Multiplexed Fiberoptic Gyro Control" in which the amount and complexity of the control electronics are simplified and reduced by multiplexing the outputs of the three fiber optic gyros to derive angular and rotation rate data as well as drive signals for the phase modulators associated with the three gyro coils by means of a single processor. In that patent a plurality of gyros is sampled at a rate of n.tau. where .tau. is the gyro optic transit time and n is an integer. The sampled signal is then fed to the digital signal processor and used to form a rate feedback signal which is then converted to analog form to drive the phase modulators. The same signal is employed to drive the phase modulators associated with the three gyro sensor coils. Accordingly, the modulators must possess identical operational characteristics. Otherwise, differences between the devices would most likely be reflected in differing scale factors that would adversely affect the accuracy of the device. Furthermore, such errors could be cumulative in nature with mounting bias errors eventually rendering the device useless.